Top
PHI - always good, always powerful. Can synergize with other traits or stand alone by itself. All PHI leaders tend to be good picks, except arguably Alex, though that's more about Greece than him.
High
CRE - free border pops are no joke for ease of settlement, help in border wars and overall speed your early game advancement. Cheap Libraries are also very very nice, compared to the cheaper Granaries of EXP, since Libraries are more expensive despite also being early. With CRE they go from "where only feasible" to "where's a spot with +4 food?" With spammed Libraries you can quickly be on your way to the first couple GS, heavily reinforce borders, or just bump your beaker rate up. It also has synergy with PHI,IMP, and IND (getting Mids is fantastic for CRE leaders). I liked this trait as newbie and love it even more now with a bit more practice, for the early game awesomeness and how it follows through, allowing quick economic setup (even recovery, by pushing to Currency with scientists while being broke), works well whether isolated or surrounded, etc.
FIN - very straightforward trait, adding +1 commerce to any 2C or higher plot. Not entirely passive as you have to utilize those tiles and purpose the commerce well; it's not a win button, but rather a cushion. The most obvious way is through cottaging, which instantly jump 50% in yield if placed on a river or 200% if worked for only 10 turns on flatland. In particular my favorite thing about FIN is how it enhances tiles you'd normally pass over and let's you get a little creative with making decent tiles to work like:
-water tiles with a Lighthouse. Even more so if they are lake tiles, and give +3f3C
-cottaging Dye/Silk for a high commerce yield tile or Banana/Sugar for high food yield cottage, if these are riverside even more impressive. Spices if green land too.
-cottaging riverside wine for an enhanced brown cottage
IND - Only trait that will directly help you with the GLH (though ORG does a bit indirectly), helps out in the race for Mids (expensive) or Great Wall (goes early) if you want to try for them, enhances Stonehenge/ToA fail gold. That's in the early game. Later you can stack huge amounts of failgold and then use it to fuel economic pushes toward things like Music, Lib, Astro, or military tech like Chemistry/Steel, etc. I also really like IND when not having marble, as I utilize National Epic regularly and try to build Taj in just about every game I can. Cheap forges are also really nice, but not the sort of thing I'd personally really rush for, just nice to have when putting infrastructure down all around in prep for war or perhaps in one high hammer city earlier (like HE city or to build wonders).
So why is it so high tier? Because of the sheer amount of fail-gold you can get with it can be mind-boggling in some games (over 2 or 3k on the right wonder), especially if compounded with a building resource like Stone, modifiers like OR and forges, or chopping aggressively, and because you can stretch raw gold further with things like Libraries. Banking on stuff like Chichen Itza, Shwedagon Paya, or Angkor Wat is usually a good bet for a few hundred gold with IND leaders resource or not, as the AIs give you a little time to work these without taking forever to complete them and they are largely worthless. Earlier fare like Parthenon and MoM can work too, as does Sistine (since SOMEONE will complete it eventually) but work better if you already have Marble to help make the most of the shorter window.
Mid
ORG - this is my personal favorite trait as I play heavily expansive a lot, and it directly helps with that by reducing civic costs (calculated by your total empire population ie. more cities = more pop = more cost) and cheapening courthouses to 2 pop whip, making them super easy to spam. Before courthouses, you can support another 2-3 cities maybe with ORG reduction before crashing compared to a non-ORG leader, making it more feasible to settle out to block off AIs, take primo spots first, add in additional help cities, etc. After courthouses you found anywhere you want (to an extent, colonial stuff still hurts). ORG is also a great war time trait as war civics tend to be more expensive, each city you take will have less economic impact, and you can cold whip courthouses in captured cities for 3 pop to instantly shave costs when it come online. Cheap lighthouses make any coastal city easy to bring online and speed the GLH (though it also removes 2pop overflow option into it). Cheap Factories is actually a really nice thing to have especially if you go to AL quickly, they can normally be quite expensive to slow build but ORG leader may roughly ~3-5turn factories in mature cities.
CHA - the real winning part of this trait is extra happy you get by default and quickly after a monument. Cities with only slightly improved happy caps are just able to do more in terms of whipping, growing to run specialists, or working cottages in the earlier stages of the game at low pops. You will also especially feel the impact of this if isolated or otherwise in a luxury resource crunch, which can cause the value of this trait to fluctuate a bit and makes it better for "harder" map setups. In particular, having this trait can make it so you don't have to crutch on Monarchy/HR or AI religion/trades as much to function, which can save you some tech detouring. The reduced unit XP for promos is decent as it applies to all units and helps get to that coveted 3-promo level, where units start to get really enhanced (Formation is 3rd level, for instance, and Pinch+Shock really enhances success when attacking AIs with Cuirs when they start picking up Muskets, etc). It should be noted that for melee or Gunpowder units AGG is actually better for faster promos.
Low
AGG - I really like this trait but it lacks broad application. AGG makes warriors better against barbs, and able to promote quickly to take Cover/Shock to make them even better especially if you build a cheap 25h Barracks. Cheap barracks removes using them for 2pop overflows though
Later on the same thing applies to Axes, enhancing any rush potential you have with them. One can also cold 2pop whip barracks for overflow as opposed to investing up 19 hammers. Being able to promote Medics at will also increases flexibility at any stage of the game, even with Rifles. Finally the free C1 promo allows faster promotion up the Combat tree until I think it was promo 7 or 8 compared to CHA. Solid trait, too bad it's completely unit focused.
IMP - Personally I am liking IMP a lot more these days for settler spamming, which can be very helpful for blocking AI settling, gifting cities for diplo or to prevent the AI settling a spot that would ruin one you want, and especially for recovering some lost turns on a poor start. If for nothing else, it allows you to quickly settle out a better spot/a few more cities to make up for a rough capitol, and I love that option.
Look how it helps me here:
I went NW looking for seafood, found none, decided 2 FP were enough with all the raw hammers + IMP, and still got 2 more cities founded by T41, 7-8 approaching T100. Quick detour into Myst to pop borders ASAP and it was off to the races with Pottery + wonder spam. It's not like, an ideal start, but Nottingham did the heavy lifting for further expansion after this and York kept it funded. Nottingham also has GLib + NE, London grabbed Mids+Parthenon+MoM+Colossus with all its raw hammers keeping it busy. A nice little building game that was prevented from being cornered in by IMP.
A really neat thing about IMP is that it makes overflow into Settlers much more effective, meaning you can manage your growth and development with bit more more flexibility, since overflowing Granaries/Lighthouses/Axes or even workers are all effective ways to quickly get a settler out too instead of growing to the target threshold of 4 or 6 and working for maximum overflow off the settler whip. Get a lot more use out of your happy threshold that way.
EXP - I do not like this trait. Health bonus is inconsequential except in the most extreme of starts, the worker bonus is weak enough at 25% that it's situational when it shaves turns or not (I also tend to just whip or overflow complete workers anyway) leaving just the Granary cheapening bonus. Thing is, Granaries are easily 2 pop whipped or 1 chop + 1 pop whipped as is, and are important enough to do just that. I don't like how the trait removes a lot of the applications of overflowing Granary whips. All that said...it DOES get the granaries up
faster, and can save turns here and there on workers, allowing for
earlier application of worker turns -- making this a PERFECT trait for whipping/chopping a rush attack out. You combine this with AGG (Shaka) or CHA (Washington) and you have an excellent early warfare potential. Other than that though, I'd rather have any of the other early game traits first.
SPI - I have said many times what I think of this trait. I understand it. I have used it. I just don't think it's that consequential. This trait, rather than direct advantages, gives you potential boons, and while the same exact thing can be said about every other trait ("it's situational"), it's never more true than with SPI. Eliminating anarchy is probably the weakest aspect of the trait other than the temple reduction, when you have learned to mitigate it yourself or to play around it (such as timing GAs). Diplomatically, one can have a field day with this trait given how the AI gives so much priority to religion, and it can be great fun to do so, but wholly unnecessary. If any of the traits most qualify as "icing on top" it's this one.
Bad
PRO - it's not that it doesn't do anything useful, it's just that what it does offer advantages for are unlikely to come up in standard play. Walls are already cheap like a Barracks, and you'll never need to have them....until you need them (seems redundant?). In fact, I hate that it removes 2 pop whipping Walls for overflow, a theme I've already expressed, as they are one of the builds you can utilize for this purpose on the GLH, Mids, or GW since all require Masonry. Castles need never be built unless you're Spain, honestly, and not least because that requires Engineering anyway besides needing to spend 75 hammers (need walls too) for a single trade route in each city. The unit promos can be fun, but even compared to AGG are more specific in application. Cho-ko-nu rush, archer choke rush with Charlie, Toku draft rifles...and that's about it? I have heard about SB Longbows/Xbows too and super machinegun defenders. Lain has demonstrated the exact tactic where this trait would be especially powerful (twice, I think), by settling a defensive city in front of a neighboring warmonger. The issue is that situation is very specific, and is also about hurting them through delay more than helping yourself. Well beyond dying, I guess, but that's a product of the situation you're already in by that point.
once your library is up). So that's a long-term benefit. In a game that really seems to be about playing the map, Creative is a trait I really feel.
everywhere. With Protective and Stone, Walls and Castles are practically free. Then, simply avoid Economics like the plague. By doing so, you can use the spare 
to get other necessary technologies, like Replaceable Parts and Rifling, and then make a beeline to Communism. (Who says you need to understand Economics in order to research Communism?) Hopefully at that point, you can just ride your Cavalry to victory. The fly in the ointment is that Engineering is ludicrously expensive to research yourself, so it's probably best to just trade for it.
bonus by going with a kind of Espionage Economy for awhile, to pick up any missing techs that you need. Of course, the 
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issues. I researched Medicine and immediately switched into Environmentalism, which markedly improved my economy due to removing all the 
could then compensate for the 
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